A device for measuring visual characteristics of surfaces and, more specifically, the reflection behavior of surfaces, has been disclosed in EP-B-0 438 468. In this known device, a light source is provided, the light of which is directed onto the surface to be measured under a predetermined angle. The light reflected from said surface under a corresponding angle is measured by a light-sensitive sensor. Thus, it is possible to detect the gloss of the surface. The known device is used to judge the gloss behavior of vehicle paintures and the like, for example.
The function of this device will be described in the following with respect to the gloss measurement:
Using this known device, only a statement about the gloss behavior of surfaces in the area is possible, where the Fresnel reflection law is valid, i. e. the angle of reflection equals the angle of incidence. Actually a diffuse reflection is superimposed on this so-called mirror reflection, said diffuse reflection leading to an appearance of said surface which is referred to as haze. For glossy surfaces having haze the contrast and the brilliance of the reflection is diminished, whereby the surface provides a high gloss value from the point of the measurement, but from the point of the human observer it is no longer judged as clear but as milky or turbid.
For the measurement of haze usually a device is used as depicted in FIG. 11. This known device comprises a tubus 1 in which a body 2 is incorporated. In this body 2 a slit 4 is provided, the lower end of which is terminated by means of a photo cell 5.
The slit 4 is covered by a plate 6, in the middle of which an aperture 7 is provided and on which a photo sensor 9 is arranged, respectively. Above said photo sensor a usual filter unit 10 is provided.
By means of this apparatus it is possible to measure a gloss value and values of the haze. At this time the angle deviation which occurs between the ideal reflection angle for exact Fresnel reflection and the angle in which the light quantity is detected by the haze sensors, amounts to approximately 2.degree..
The function of this usual device is as follows: The surface to be measured is illuminated by a not illustrated light source, and an optic is provided to guide the reflected light into the tubus illustrated in FIG. 10. In this case the direction of the incidence of light exactly runs in parallel to the longitudinal axis 12 of the tubus. The light which is straightly reflected into the direction of the angle of reflection passes through the slit 7, which is the gloss aperture, onto the photo sensor 5. The intensity of the incident light quantity is a measure of the reflection capability of the surface and thus for its gloss.
The light which is reflected from the surface under a slightly smaller or larger angle, does not impinge onto the gloss aperture 7, but impinges partly onto the photo sensors 9. Their area is limited by edges 14 and by the slit 7 of the gloss aperture.
The intensity of the light impinging onto the sensors 9 is a measure of the haze. For an exactly reflecting surface, for example a mirror, the total reflected amount of light passes through the slit 7 onto the photo sensor 5, and no light impinges onto the photo sensors 9. For a highly brilliant surface exhibiting haze, however, a larger amount of the light impinges onto the sensors 9.
This known apparatus exhibits the disadvantage that the manufacturing accuracy has to be high in order to obtain meaningful measurement results. In this respect, especially the gloss aperture and the edges 14 which limit the impinging area of the photo sensors at the sides, have to be very accurately manufactured. Particular difficulties arise if the apparatus should be manufactured with small size, for example to be applied as a manual apparatus in the production supervision or the like. In this case the dimensions of the aperture is decreased and the manufacturing efforts are correspondingly enhanced.